"McGuinness used to be a senior member of the IRA, the group that killed the queen's cousin, Lord Louis Mountbatten in 1979. ... The handshake signals times have greatly changed since the end of the conflict, which claimed more than 3,500 lives, though some tensions remain."

McGuinness is now a deputy first minister in Northern Ireland's government. The queen is on a visit to Northern Ireland. McGuinness told the BBC it was "very nice" to meet her. And the BBC adds that according to a spokesman for McGuinness' Irish nationalist party, Sinn Fein, McGuinness told the queen that their meeting was a "powerful signal that peace-building requires leadership."

The BBC's Ireland correspondent, Mark Simpson, says today's handshake will not be the queen's "favorite moment of her 60-year-reign, but it is certainly one of the most significant."

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